Im stuck with a choice of 27t or 42t on the front ring if my 1×9 setup. Are they both unsuitable? I live in a fairly flat area but occasionally will do a big trail centre day out so will need a smaller ring. My cassette is a 12-36 (ish).

Will have to go 2×9 otherwise and don’t really want to.

My 2 x 9 is std 32/11 rear 26/36 front no probs in a hilly area covering trail centres and Scottish stanes with the most being in the 36. My new build is a 10sp single 34 up front 34/11 rear I am hopeing it will be ok. I don’t mind very occasional pushing but it has to be justifiable – I have yet to find out. 42 ??? thats big all you can do is try it out and see what works for you.
Good luck

Thanks tony, think I will go for the 27 but I just haven’t heard of anyone else trying it so I still have doubts! If you manage in a hilly area with that setup maybe I’ll be ok.

Sorry to say I wouldn’t bother, 27 tooth ring up front will leave you madly spinning away on anything remotely down and 42 tooth will leave you (well would leave me) pushing up anything remotely up. Can you not just order a 32 or 34 tooth ring to replace the two?

I’m massively unfit and run a 32 up front with a 10 speed 12:36 out back (I think) and that seems bout right for most riding around Surrey hills, swinley and the peaks.

Is 2×9 really that bad??

Neither would suit me- 27 too low, 42T quite a lot too high. 27 would be least bad, but still bad.

I used to use 32 on an 11-34 cassette, that worked alright. Not quite low enough and not quite high enough but fairly balanced at least. But ymmv, depends on so many things- fitness, where you ride, etc.

33t works for me.

It’s an awkward bloody 3 bolt chainring, should have mentioned that. Would be happy with 32ish up front but you can’t get them anywhere

FSA by any chance? Personally I’d leave it as 2×9 and look at new cranks if it really bothers you

^^ this

Neither are gonna work properly. New cranks if you want 1×10.